using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Entrer un numero");

        int a = int.Parse(Console.ReadLine());

        int doubledNumber = a.Multiple();

        Console.WriteLine(doubledNumber);

        Console.ReadKey();

    }

}

​

public static class Extensions

{

   public static int Multiple(this int a)

   {

       return a * 2;

   }

}